The lipooligosaccharide from Neisseria gonorrhoeae (GC), consists of lipid A, an oligosaccharide core and three branches, alpha, beta, and gamma. We report the cloning of the gene (lgtG, lipooligosaccharide glycosyl transferase G) encoding the glucosyl transferase of GC that initiates the chain which consists of a lactosyl moiety. This gene contains a homopolymeric tract of cytidine [poly(C)] and we demonstrate that changes in the number of Cs in poly(C) account for the variation of chain expression in different GC strains. Biochemical analyses and mass spectrometry clearly attribute the reactivity of mAb 2C7 to the presence of the lactosyl chain. In addition, we demonstrate that in the absence of the lactosyl group, a phosphoethanolamine is added to generate a new antigenic epitope as evidenced by the gain of reactivity to mAb 2-L1-8. These results show that, like the chain, the chain of lipooligosaccharide is subject to antigenic variation. Asesh Banerjee, Rong Wang, Sacha N. Uljon, Peter A. Rice, Emil C. Gotschlich, and Daniel C. Stein (1998) PNAS USA 95, 10872-10877.